1. Technical Field
The present disclosure generally relates to operational amplifiers used in electrical and electronic circuitry and, in particular, to ways of adjusting operational amplifiers to compensate for variations in performance that arise in certain environments.
2. Description of the Related Art
Operational amplifiers (“op-amps”) are general purpose differential voltage amplifiers commonly used in electric circuits for many different applications. For example, op-amps are frequently used as building blocks in audio systems that involve complex signal processing circuitry to amplify sound. An idealized op-amp 100, shown in FIG. 1A, is a three-terminal device that has two input terminals, V+ (a non-inverting input), V− (an inverting input), and one output terminal, Vout. (Additional terminals are provided to connect a power source (e.g., battery) at Vs+ and Vs−). The differential input voltage to be amplified is ΔV=V+−V−. Components internal to the idealized op-amp 100 cause the differential input voltage ΔV to appear at the output terminal, multiplied by an amplification factor, or gain, so that Vout=AΔV. Amplification factors for op-amps can be in the range of about 1000-100,000. This characteristic makes op-amps suitable to be used as sensors in many applications in which changes in a small signal can be exaggerated so the changes are easier to monitor and detect.
If a fixed reference voltage Vref is applied to the non-inverting input, the op-amp acts as a comparator device 150, shown in FIG. 1B, that compares an input voltage at the inverting input− to Vref. If the input voltage Vin is about equal to Vref, the amplified output Vout remains small. As Vin deviates slightly from Vref, the deviation is amplified so that Vout becomes very large and can serve as a trigger signal to a downstream device.
One way to test the accuracy of an op-amp used as a sensor or a comparator is to deliberately set the differential input voltage ΔV to zero (for example, by connecting the two inputs together, or grounding both of the inputs), and verifying that Vout is also zero. However, intrinsic errors in the various internal components of the op-amp can compound and cause a detectable zero error referred to as an input offset voltage Vos. If a non-zero potential Vos exists across the two inputs, Vos itself can be amplified, producing a significant erroneous non-zero voltage at the output. The zero error associated with an op-amp can be thought of as analogous to the zero error associated with a scale, which is evident when the scale registers a non-zero weight prior to an object being placed on the scale. Scales typically come equipped with mechanical zeroing adjustments to correct these zero errors. In a similar fashion, the input offset voltage Vos of an op-amp can be corrected, or nullified, by electrically coupling an “offset null” stage to the V− input of the op-amp.
FIG. 2 shows one example of an existing voltage-compensated op-amp 200 in which a conventional op-amp 202 that exhibits an input offset voltage Vos features such an offset null stage 204. In this example, the offset null stage is in the form of a voltage source Vnull that effectively zeroes out the erroneous input offset voltage. The input offset voltage Vos to be nullified is typically within the range of about a microvolt (μV) to about a millivolt (mV). The offset null stage 204 can take other forms, such as, for example, one or more of a fixed resistor, variable resistor, potentiometer, current source, or thyristor. If the input offset voltage Vos is known to be constant, an offset null having a constant resistance (e.g., a fixed resistor or a network of fixed resistors) can be applied. If the input offset voltage Vos fluctuates, causing a ΔVos, an offset null including an adjustable resistance, or a potentiometer, can be applied. The offset null can be applied at one or both of the inputs to the op-amp, or the offset null can be provided as a built-in offset null stage that is internal to the op-amp.
FIG. 3 shows another embodiment of a voltage-compensated op-amp 300, in which a conventional op-amp 302 (with its internal components shown), that exhibits an input offset voltage, is provided with a built-in offset null stage 304.
With reference to FIG. 4, a conventional op-amp 400 is shown in which there exists a difference between the currents at the two inputs, I+ and I−, referred to as an input offset current, Ioffset=|I+−I−|. The average of the two input input currents is referred to as an input bias current: Ibias=(I++I−)/2. In an ideal op-amp, the input resistance is so high that the input currents I+ and I− are both negligible, and therefore Ibias=0. Any current appearing at the input terminals of the op-amp 400 is then a small leakage current which is negligible in most applications, even those in which attention is paid to cancelling an input offset voltage. Input bias currents typically range from about a micro-amp to as small as a pico-amp. In some applications, however, a nonzero input bias current Ibias brings about another environmentally-dependent, fluctuating source of error, ΔIbias, that warrants cancellation. Therefore, when an input bias current is present, use of an offset null stage to simply cancel Vos (or ΔVos) may be insufficient. An input bias current can be nullified by addition of resistors to the circuit, for which the resistor values are preferably carefully selected to precisely cancel the bias current without otherwise affecting circuit performance.